GENERAL GROWTH OF THE EMHRYO. 



C'C. 



considerably narrower, while the groove along the axis becomes shal- 

 lower and disappears. The anterior, and at first proportionately a 

 very large part, soon becomes distinguished as the cephalic region 

 (fig. 33 B). The medullary cord in this region becomes very early 

 divided into three indistinctly separated lobes, representing the fore, 

 the mid, and the hind brains : of these the anterior is the smallest. 

 With it are connected the optic vesicles (oc) solid at first which 

 are pushed back into the region of the mid-brain. 



The trunk grows in the usual way by the addition of fresh somites 

 behind. 



After the yolk has become completely enveloped by the blastoderm 

 the tail becomes folded off, and the same process takes place .at the 

 front end of the embryo. The free tail end of the embryo continues 

 to grow, remaining however closely applied to the yolk-sack, round 

 which it curls itself to an extent varying with the species (vide 

 fig. 34). 



The general growth of the embryo during the later stages pre- 

 sents a few special features of interest. 

 The head is remarkable for the small 

 apparent amount of the cranial 'flexure. 

 This is probably due to the late de- 

 velopment of the cerebral hemispheres. 

 The flexure of the floor of the brain 

 is however quite as considerable in 

 the Teleostei as in other types. The 

 gill clefts develop from before back- 

 wards. The first cleft is the hyoman- 

 dibular, and behind this there are the 

 hyobranchial .and four branchial clefts. 

 Simultaneously with the clefts there 

 are developed the branchial arches. 

 The postoral arches formed are the 

 mandibular, hyoid and five branchial 

 arches. In the case of the Salmon all 

 of these appear before hatching. 



The first cleft closes np very early (about the time of hatching in 

 the Salmon) ; and about the same time there springs a membranous 

 fold from the hyoid arch, which gradually grows backwards over the 

 arches following, and gives rise to the operculum. There appear 

 in the Salmon shortly before hatching double rows of papilhe on 

 the four anterior arches behind the hyoid. They are the rudiments 

 of the branchia?. They reach a considerable length before they 

 are covered in by the opercular membrane. In Cobitis (Gotte, 

 No. 64) they appear in young larvae as filiform processes equivalent 

 to the external gills of Elasmobranchs. The extremities of these 

 processes atrophy; while the basal portions become the permanent 

 gill lamella. The general relation of the clefts, after the closure of 

 the hyomandibnlar, is shewn in fig. 35. 



FlG. 34. \IK\V OF AN ADVANCED 

 EMBRYO OF A HERRING IN THE EGG. 



(After Kupffer.) 



oc. eye; )it. heart; 7(?/r. post- 

 anal vesicle; ch. notochord. 



